This invention relates to displacement detecting apparatus and, more particularly, to apparatus of this type in which digital signals are generated when relative displacement between an object and a sensor exceeds a predetermined incremental amount.
In the control of, for example, a machine tool, it is important to know the displacement of that tool relative to a workpiece. More accurate control over the tool is attained if the control apparatus therefor is a digital control system. Consequently, it is advantageous to provide displacement detecting apparatus which produces displacement-related signals in digital form.
One type of displacement detecting apparatus is described in U.S. Pat. No. 3,582,924, assigned to the assignee of the present invention. In the earlier apparatus, a member, whose displacement is to be detected, is provided with a magnetic scale upon which reference divisions, known as magnetic gratings, which serve as calibrating signals, are recorded. The magnetic gratings exhibit a predetermined wavelength, and a pair of magnetic heads are disposed to reproduce such magnetic gratings, which appears as sinusoidal signals whose amplitudes vary as a function of the displacement of a magnetic grating relative to the magnetic heads. The heads also are energized with an oscillating signal of predetermined frequency, which signal serves as a carrier to be modulated by the reproduced magnetic gratings. The output signal thus produced by each head is a sine (or cosine) function with an envelope also varying as a sine (or cosine) function. That is, the output signal produced by each head appears as a balanced modulated signal.
In accordance with the apparatus described in the aforementioned patent, the two balanced modulated signals produced by the magnetic heads are mixed to produce a phase-modulated signal whose carrier frequency is equal to the carrier of each of the balanced modulated signals. This phase modulated signal then is shaped to a pulse form, and the phase variations of this pulse signal are detected by comparing its phase to the phase of clock pulses, and the thus-detected phase shift is measured to obtain an indication of the amount of displacement.
For accurate digital control over the machine tool, displacement should be measured with high resolution. That is, the smallest incremental amount of displacement which can be sensed should be made as small as possible. For example, if the wavelength of the magnetic gratings is on the other of about 100 microns, it is desirable to measure a displacement as small as 10 microns. This means that the frequency, or repetition rate, of the clock pulses which are used to detect phase modulations should be much greater than the frequency of the phase modulated pulse signal. However, this relationship between the clock and phase modulated pulse signals preferably should not be attained by lowering the frequency of the phase modulated pulse signal because this would undesirably reduce the overall speed of response of the displacement detecting apparatus. Accordingly, depending upon the resolution which is required, displacement detecting apparatus of the aforedescribed type has been proposed wherein the frequency of the phase modulated pulse signal is on the order of 10 to 50 KHz. But, when this frequency is selected, the filter which is used to eliminate unwanted higher frequency signals from the phase modulated signal which is produced by adding the balanced modulated signals derived from the magnetic heads is a passive filter formed of coils and/or capacitors. This results in a bulky and expensive filter which is a significant disadvantage.
The aforementioned disadvantage can be overcome if the frequency of the phase modulated signal is made higher, so that the filtering frequency of the filter likewise can be made higher. It has been proposed to frequency convert the phase modulated signal produced in response to the mixing of the balanced modulated signals obtained from the magnetic heads to a higher carrier frequency. This can be achieved by connecting a balanced modulator to receive the phase modulated signal and by supplying this balanced modulator with a relatively high frequency carrier. The output of the balanced modulator thus is a phase modulated signal of relatively high frequency. Then, the filter, such as a band pass filter, through which this higher frequency phase modulated signal is passed can be a lower cost ceramic filter. The filtered phase modulated signal can be phase detected by supplying it to a phase comparator together with a clock signal whose frequency is the same as the frequency of the carrier which is supplied to the balanced modulator. The output of the phase comparator then may appear as a phase modulated pulse signal of a relatively low repetition rate. Hence, the clock pulses which are used to measure this phase modulation may exhibit a desirably lower frequency.
In the aforementioned apparatus, the magnetic heads which are used to produce the balanced modulated signals are saturable magnetic heads. If these heads are supplied with energizing signals of a frequency equal to, for example, f.sub.c /2, the output of the head will include a component whose frequency is equal to f.sub.c /2, another component whose frequency is equal to f.sub.c, and higher harmonics. In general, the saturable head suppresses the component of frequency f.sub.c /2 and is sensitive to the carrier frequency of f.sub.c. The balanced modulated signals, each of carrier frequency f.sub.c, after being mixed to produce a phase modulated signal, also of frequency f.sub.c, are frequency converted by a carrier frequency of, for example, nf.sub.c to a frequency-converted phase modulated signal whose frequency either is equal to (n-1) f.sub.c or (n+1)f.sub.c, depending upon the center frequency of the band pass filter. This phase modulated, frequency-converted signal of carrier frequency (n-1)f.sub.c or (n+1)f.sub.c is phase-detected, or demodulated, by a signal of frequency nf.sub.c, so as to obtain a phase-modulated component of frequency f.sub.c. It is desirable to obtain a phase modulated signal of the relatively higher frequency (n-1)f.sub.c or (n+1)f.sub.c with a simplified circuit configuration. In particular, it would be advantageous to obtain such higher carrier frequencies directly at the outputs of the magnetic heads without requiring the use of a balanced modulator.